The present invention relates to a brake pressure control means that reduces or eliminates hysteresis effects in hydraulic braking systems for vehicles in which a controller reduces brake pressure on the rear axle as a function of static and load distribution referred to the front axle.
Federal Republic of Germany Patent Application OS No. 16 80 818 discloses a brake-pressure control system in which pressure in the brake circuit for the rear axle increases by a reduced amount as compared to pressure in the brake circuit for the front axle. This pressure difference is obtained by operating a control valve and results in an input-output pressure diagram for the rear axle that is a bent line during braking. Upon the reduction of braking pressure, the control valve remains closed until the pressure in the unreduced (front) brake circuit is equal to the pressure in the reduced (rear) brake circuit. In the input-output pressure diagram, the return curve for the rear brake circuit is initially a straight line parallel to the axis and then moves along the unreduced excess pressure line for the front circuit. Since the return curve for the rear axle does not correspond to the pressure increase curve, hysteresis results, so that the rear axle is over-braked initially while braking pressure is reduced.
Federal Republic of Germany Patent Application OS No. 17 80 560 discloses a system that attempts to avoid the disadvantages described above by providing a storage space which receives excess fluid from the rear axle circuit upon a reduction of braking pressure. This requires that the stroke of the master cylinder be of increased length and, upon multiple actuations of the brake pedal in a manner such that there is pressure buildup and reduction without complete relief, the capacity of the storage space is soon exhausted.
For brake systems in general, and particularly in those brake systems that are provided with antilocking devices, it is, however, desirable that the course of the return characteristic curve correspond to the course of the characteristic curve upon the buildup of the pressure so that no hysteresis is present.